69goose, I would avoid any kind of Nuts or glue on the inside of the Cover.
I suggest to drill a 4,5mm Hole in each corner of the Signs, then attach the Signs on to the Covers with some tape and mark the Holes on to the Covers. Then PUNCH a Hole through the Sheet Metal Cover with at very sharp Awl or similar so that the diameter of each hole is 3,5mm. That will allow you to take a 4mm Tap and cut a Thread in each Hole and because you have punched the Holes there will be a really good basis for a Thread and subsequently attach each Sign with 4mm Screws in the Corners (use a drop of Locktite to seal). I suppose Australia is Metric land or you can alternative use a similar size Imperial
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